Over its somewhat abbreviated history, the computer industry has developed a wide range of storage techniques for data, now generally present in bit form as a flux reversal in magnetic media. Some applications storing mass volumes of data involve equipment which is correspondingly large and accessing or retrieving the data can be at a relatively leisurely pace. Recent emphasis, however, has been to the enhancement of the data retaining and accessing capability of small computers employing hard disk drives for primary data retention. Typically, the smaller of these disks are less than a nominal 8 inch diameter. The small disks perform in conjunction with read/write heads configured as "sliders" supported by relative air velocity about 5 micro-inches above the magnetic media surface. Positioning of the head with respect to a given one of the concentric tracks on a disk is by a head actuator.
Head actuators traditionally have taken two general forms, linear and rotary. The linear actuator moves a head along a locus aligned with a disk radius. This movement is provided by a head support assembly mounted upon bearings and driven by the coil of an actuator configured, for example, as a voice coil assembly. Such linear devices generally perform as Faraday based systems, the coil moving under the relationship F=Bli where F is force, B is field strength, 1 is the coil wire length that is inside of and orthogonal to the magnetic field, B, and i is the current within such length of wire, 1.
Rotary head actuators generally are called for in the case of smaller disk systems. These actuators typically are fashioned as a dual sided pivot arm, pivotally movable about bearings mounted, in turn, upon a fixed pivot shaft. One side or component of the arm carries the head, while the opposite side is driven in fulcrum based fashion about the shaft. A voice coil form of drive usually provides the involved pivotal drive. The disk drive manufacturing community has tended to favor the use of rotary actuators because of lower manufacturing costs and the ease of their assembly.
The computer industry is now and has been placing expanding emphasis upon disk drive systems exhibiting (a) maximum data throughput at minimum power and (b) higher data storage capacity at least cost per bit of stored data for a storage device of given size. A major aspect in improvement of the above factors is concerned with density on the disk. Such bit density is the product of the flux reversals per inch of track times the number of tracks per inch (TPI). At the outset of "Winchester" technology, flying heads or sliders performed in conjunction with densities of about 500 TPI. In short order, densities of 1,000 TPI became conventional and densities in the range of 3,000-5,000 TPI are contemplated as feasible, given that investigators will solve a variety of posed technical problems.
While bearing problems associated with the spindle motors rotating disks have been an important limitation in the evolution of track density, solutions to those problems are considered to be on their way. The second, most serious limitation regarding TPI or bit density has to do with the ability of the head positioning actuator to move to a new position quickly at low power and to be mechanically stable after its arrival.
One aspect of track positioning inaccuracies encountered as bit densities increase and track widths become smaller resides in the movement or flexure of bearings and shaft as an actuating force is imposed upon the actuator assembly including the bearing components thereof and is opposed by reaction force at the pivot shaft. For example, where one side of a rotary head positioner is subjected to a force to cause a rotation about a pivotal shaft or fulcrum, that force will be reflected in an opposite direction at the shaft through the bearings. These bearings represent a spring system which also is subject to that force. Generally, ball bearings are used for these fulcrum structures because of the very high stiffness yet low friction that can be attained where sufficient pre-load is provided to the bearing assembly. The ball bearings act as a spring with a high spring rate, the higher the pre-load the higher that spring rate. However, the higher the pre-load, the shorter the life of the bearing and the sooner head positioning anomalies will occur. As pre-loading becomes greater to accommodate for tracks of smaller widths, as measured in microns, bearing support flexure can easily alter head position by one-fourth of the track width or more. Because small disk drive based computers very often are being carried by users in mobile environments, present industrial specifications call for no more than a 20% displacement of a track width during the imposition of a 10 g side load upon the structure. Such specifications are expected to become even more stringent. This calls for both higher preloading and static balance. As pre-load is increased on the tiny bearing structures, the arm components begin to be difficult to move, the actuator components themselves having very little power. Additionally, a highly pre-loaded bearing will often exhibit friction or "stiction". In the case of the latter phenomena where linearly increasing current is applied to the actuator, arm movement does not take place. As the current is further increased to create greater rotational force, the arm will "break loose" as it were, to move quickly from its stuck position and exhibit entirely unpredictable and unacceptable performance. This stiction phenomena is increasingly important as the actuator components become smaller; with proportionately lower torque, as the market trend toward decreasing disk drive size continues. As is apparent, one important aspect in the evolution of improved small disk drives resides in a solution to these induced side forces at the bearings of the pivot structure of rotary head actuators.